Receive the latest msn updates in your inbox

On Wednesday, 29-year-old Carlo Mercado pleaded not guilty to three counts murder in a case that started last Christmas Eve. NBC 7’s Bridget Naso reports. (Published Wednesday, Jun 25, 2014)

Updated at 6:14 AM PST on Thursday, Jun 26, 2014

The man accused of killing three San Diegans -- shooting two of his victims in mall parking lot on Christmas Eve and leaving his third victim dead in the trunk of his own car -- could face the death penalty or life in prison, according to prosecutors.

He was arraigned on three first-degree murder charges at the San Diego County Courthouse Wednesday and entered a not guilty plea. In court, he appeared disheveled and despondent, keeping his head hung low. He only uttered a few words when prompted by a judge.

The San Diego District Attorney’s Office filed a three-page complaint Wednesday morning formally charging Mercado with the three murders.

Mission Valley Mall Christmas Eve Shooting: Images

The complaint states that Mercado killed Flint and Salvatore “on or about Dec. 24, 2013,” and killed Gianni “on or about and between Dec. 23, 2013, and Jan. 17, 2014.” Those dates mean Gianni could have been killed before Ilona and Sal, but at this point, that remains unknown.

Each murder charge against Mercado also includes a special allegation for the use of a handgun in the murders, according to the document.

Each count carries a sentence to 25 years to life in state prison. The complaint also indicates that Mercado is eligible for the death penalty in this case if he is convicted of more than one murder. The district attorney will decide before trial whether or not to seek the death penalty or life in prison.

If Mercado is then convicted of more than one murder, the jury will decide whether he gets life in prison or the death penalty.

VIDEO: Chief Announces Arrest in Triple Murder

At a news conference Monday, San Diego Police Chief Shelley Zimmerman announced the 29-year-old Carlo Mercado was arrested in a triple murder that has made headlines since Christmas Eve. The chief expressed her sorrow the family and applauded her officers for their passion to seek justice. (Published Monday, Jun 23, 2014)

Deputy District Attorney Brian Erickson said that because there are three victims in this case, it has reached capital status. As a result, Mercado waived his right to argue bail.

In the early hours of Dec. 24, 2013, Flint and Salvatore were gunned down in a parking lot outside a Macy’s department store at Westfield Mission Valley Mall. Flint was shot in the head, but somehow managed to call 911 and report the shooting. She died at the scene.

Salvatore died a few days later from injuries sustained in the shooting.

Gianni – Salvatore’s brother and Flint’s longtime boyfriend-turned-fiancé – went missing immediately following the shooting of his loved ones. The San Diego Police Department launched a missing person search for Gianni over the next several weeks.

Finally, on Jan. 17, 2014, Gianni's body was discovered in the trunk of his car in Riverside, Calif., about an hour-and-a-half north of San Diego County. He, too, had been fatally shot, also deemed the victim of a homicide.

Until Mercado’s arrest over the weekend, a suspect had not been caught in connection with the killings.

At a news briefing on Monday, SDPD Homicide Lt. Mike Hastings said Mercado’s arrest came after investigators and forensic lab personnel developed substantial probable cause to suspect Mercado in the slayings.

Hastings said a dedicated team of homicide investigators had worked tirelessly over the last six months to track down the killer in this case. SDPD Chief Shelley Zimmerman said she hoped Mercado’s arrest would begin to bring some sense of closure to the families of the victims.

Berg also confirmed Mercado was previously arrested earlier this year on four gun-related charges. Public records show Mercado had pleaded guilty, with his sentencing for those charges scheduled for July 9.

Following his arrest for the triple homicide case, Mercado was hospitalized due to an unspecified incident that happened while in custody at San Diego Central Jail. He was treated under guard at UC San Diego Medical Center on Sunday.

As of Tuesday night Jan Caldwell of the San Diego County Sheriff’s Department confirmed that Mercado had been relocated from the hospital and was back at San Diego Central Jail, awaiting his arraignment.

Though Mercado is under arrest, many questions remain unanswered in this case.

Investigators have not commented on a possible motive for the killings. Erickson said some of those details would be revealed at a later time, during a preliminary hearing.

Hastings said that because the case has now been turned over to the San Diego District Attorney’s Office, the police department can no longer comment on the investigation.

Erickson said prosecution on this case will be a lengthy process and prosecutors have braced the families of the victims for this.

For his part, Berg said his client maintains his innocence.

“He’s very dismayed, to say the very least, and extremely depressed about his circumstances of being in custody,” Berg told reporters following Mercado's arraignment.

“Right now we have a gentleman that’s been charged in a triple homicide but at this point in time we don’t know any of the facts and circumstances of this case. There’s been no motive that’s been alleged, there’s nothing that’s come out that’s tied him to these homicides,” he added.